RNA interference (RNAi) is a post-transcriptional, targeted gene-silencing technique that uses double-stranded RNA (dsRNA) to degrade messenger RNA (mRNA) containing the same sequence as the dsRNA. The process of RNAi occurs naturally in a cell when ribonuclease III (Dicer) cleaves the longer dsRNA into shorter fragments called siRNAs. The smaller RNA segments then mediate the degradation of the target mRNA. Dicer has also been implicated in the excision of small temporal RNAs (stRNAs) from precursor RNA of conserved structure that are implicated in translational control. Hutvagner et al. 2001 Science 293: 834. The RNAi response also features an endonuclease complex, commonly referred to as an RNA-induced silencing complex (RISC), which mediates cleavage of single-stranded mRNA complementary to the antisense strand of the siRNA. Cleavage of the target RNA takes place in the middle of the region complementary to the antisense strand of the siRNA duplex.
siRNAs naturally generated by Dicer typically comprise two 21-nt RNA strands, which have a 19-bp duplex region and two dinucleotide overhangs. See, for example, Elbashir et al. 2001 Nature 411: 494-498; Elbashir et al. 2001 EMBO J. 20: 6877. This is the so-called “canonical” structure of siRNAs.
There exists the continuing need to develop novel artificial structures which have improved activities, e.g., increased RNA interference activity, increased duration of activity, increased resistance to nuclease degradation and/or increased specificity.